1. Field of the Invention
The present invention relates to organic electroluminescence devices (which may hereinafter referred to as organic EL devices) and polymeric fluorescent substances used therein. More particularly, the present invention relates to organic EL devices with high luminous efficiency and polymeric fluorescent substances used therein, having a strong fluorescence and being soluble in solvents.
2. Description of the Related Art
Inorganic electroluminescence devices (which may hereinafter be referred to as inorganic EL devices) using an inorganic fluorescent substance as the light emitting material have been applied to various uses, for example, as a flat light source for backlight of liquid crystal displays and various display devices such as flat panel display and the like. In these devices, however, a high-voltage alternating current has been required for driving the devices.
Recently, Tang et al. manufactured an organic EL device having a double-layer structure comprising a laminate of a light emitting layer made of an organic fluorescent dye and a layer of an organic charge transport compound generally used, for example, in a photosensitive layer for electrophotography to realize a low-voltage-drive, high-efficiency and high-luminance organic EL device (JP-A-59-194393). In view of many advantageous features of organic EL devices as compared with inorganic EL devices, such as low-voltage drive, high luminance and easy luminescence of a large number of colors, various attempts have been made and reported regarding the development and improvement of the device structure, organic fluorescent dye and organic charge transport compound used in organic EL devices [Jpn. J. Appl. Phys., Vol. 27, L269 (1988); J. Appl. Phys., Vol. 65, p. 3610 (1989)].
Hitherto, low-molecular weight organic fluorescent dyes have generally been used as a material of the light emitting layer and, regarding polymeric light emitting materials, proposals have been made in some patents and publications such as WO9013148, JP-A-3-244630 and-Appl. Phys. Lett., Vol. 58, p. 1982 (1991). WO9013148 discloses in the Examples an EL device using a thin film of poly(p-phenylene vinylene) obtained by forming a film of a soluble precursor on the electrode and subjecting it to a heat treatment to convert the precursor into a conjugated polymer.
JP-A-3-244630 illustrates conjugated polymers having a salient feature that they are themselves soluble in solvents and make heat treatment unnecessary. In Appl. Phys. Lett., Vol. 58, 1982 (1991) are also disclosed polymeric light emitting materials soluble in solvents and organic EL devices made by using such materials. Most of the organic EL devices manufactured by using these materials, however, were not sufficiently high in luminous efficiency.
An attempt to increase the quantum yield of fluorescence of polymeric light emitting material is already reported in Nature, Vol. 356, p. 47 (1992). In the literature, it is described that by, in the course of forming a conjugated segment from a soluble precursor copolymer by heat treating, allowing the non-conjugated segment of the copolymer to remain unchanged, there can be obtained a polymer having both a conjugated segment and a non-conjugated segment and emitting a strong fluorescence. Here, since the 2,5-dimethoxy-p-phenylene-methoxy-ethylene moiety in the copolymer is hard to decompose by heat treatment alone, it is utilized for inhibiting the copolymer from being converted in its entirety into a conjugated polymer by the heat treatment.
It is further reported that a polymer in which a conjugated monomeric moiety with strong fluorescence and an aliphatic hydrocarbon are coupled via an ether linkage, generates a blue fluorescence because of its short conjugated chain length, and a blue light-emitting organic EL device can be made by using said polymer [Macromolecules, Vol. 26. p. 1188 (1993)].
It is also reported that an alternating copolymer composed of unsubstituted p-phenylene vinylene and unsubstituted m-phenylene vinylene has a greenish blue fluorescence [Vysokomolekul. Soedin., Vol. 5, p. 805 (1963)]. However, the quantum yield of fluorescence and EL properties of said copolymer are unknown.
With respect to the luminous efficiency of organic EL device, it is reported that since a poly-p-phenylene vinylene derivative (in which a cyano group is introduced into the vinylene group of p-phenylene vinylene) having a red fluorescence has high electron affinity and enables easy electron injection thereinto, said derivative allows for the production of an organic EL device which is high in luminous efficiency, i.e. the proportion of photons released per one electron injected [Nature, Vol. 365, p. 628 (1993)].
However, in the manufacture of organic EL devices using polymers reported hitherto, it has been necessary to mold a soluble precursor into a thin film and convert its structure into a conjugated polymeric structure by high-temperature heat treatment, so that there have been restrictions on the materials used for the associated parts such as substrate and the like in said manufacture. Also, when a precursor polymer is converted into a conjugated polymer by heat treatment, control of the non-conjugated moiety in the latter polymer has been made by adjusting the heat treatment conditions, so that such control has not been perfect and, when the conjugated polymer was used for an organic EL device, the device had, in long-time continuous drive, a fear of change in the structure owing to the heat generation of the device or other factors.
When there is used a polymer in which a conjugated monomeric moiety is coupled with a non-conjugated aliphatic hydrocarbon via an ether linkage, the polymer has a short conjugated chain (the length of conjugated chain is thought to contribute to transport of charge) and thereby the transfer of charge in the polymer is feared to be difficult.
In the case of soluble conjugated polymers, although no high-temperature heat treatment is necessary after formation of the resin film, further improvements are required on the quantum yield of fluorescence as well as on the luminous efficiency of the EL devices using said polymers.
As to the hitherto known poly-p-phenylene vinylene derivatives in which a cyano group is introduced into the vinylene group of the p-phenylene vinylene, no detail is disclosed on the quantum yield of fluorescence of said polymers.
Thus, there have been desired a polymeric fluorescent substance which has excellent solvent solubility, high quantum yield of fluorescence and excellent electrical conductivity; and an organic EL device of high luminance which can easily be produced using said polymeric fluorescent substance by coating.
The objects of the present invention are to provide a polymeric fluorescent substance having high quantum yield of fluorescence, excellent solvent solubility and excellent charge transport; and an organic EL device of high luminance and high luminous efficiency which can easily be produced using said polymeric fluorescent substance by coating.